Update: Please note there is a more recent version of this video here using the brand new PRIORVALUE functionality (Feb 2021).
Watch this video where I walk you through step by step as I create a record-triggered before-save flow and use workarounds to utilize ISCHANGED/ISNEW functionality. This video was recorded using a Winter 21 preview instance. So the short answer to the title question is that we still cannot use ISCHANGED/ISNEW, but the long answer is that we can use workarounds to get this functionality in a before-save flow. And all this became even more efficient in Winter 21.
Please note that there is an error in logic I discovered after I recorded this video. I needed to add to the FromAnotherStage decision outcome condition ‘AND PrevStage Does Not Equal to Closed Won’ to allow for NotChanged to work correctly. Since I don’t do anything under NotChanged condition it did not affect the outcome here.
Please see the correct version here:
Please read this blog post for an example on Criteria Enhancement and Advanced Decision Elements in Winter 21 Release.
BRDPro Consulting 
Can we do the same in after update record triggered flow??
LikeLike
When you do an after-save flow, you currently do not have access to the priorvalue. You can use enhanced entry criteria and advanced decisions to check if a field has been changed to meet your criteria, which may give you the functionality you want. Check this out: https://brdpro.com/2020/09/25/learn-decision-elements-in-winter-21-flows-video/
LikeLike